The present invention is directed to a power controller that supplies a specified power to a load, and more particularly to a voltage converter for a lamp that converts line voltage to a voltage suitable for lamp operation.
Some loads, such as lamps, operate at a voltage lower than a line (or mains) voltage of, for example, 120V or 220V, and for such loads a voltage converter that converts line voltage to a lower operating voltage must be provided. A lower lamp operating voltage provides benefits, including the ability to use a filament wire with a larger cross section and shorter length, thereby increasing lamp efficiency, color temperature and filament efficacy, and improving sag resistance, hot shock resistance and vibration induced flickering.
The power supplied to the load may be controlled with a phase-control clipping circuit that typically includes an RC circuit and uses phase-control clipping to reduce the line voltage to an RMS load voltage suitable for operation of the light emitting element of the lamp.
A simple four-component RC phase-control clipping circuit is shown in FIG. 1 and includes a capacitor 22, a diac 24, a triac 26 that is triggered by the diac 24, and resistor 28. The resistor 28 may be a potentiometer that sets a resistance in the circuit to control a phase at which the triac 26 fires. The RMS load voltage and current are determined by the resistance and capacitance values in the clipping circuit since the phase at which the clipping occurs is determined by the RC series network and since the RMS voltage and current depend on how much energy is removed by the clipping. FIG. 2 illustrates a clipped waveform formed by a phase-control clipping circuit such as illustrated in FIG. 1.
The phase-control clipping circuit has drawbacks. As seen in FIG. 2, the voltage is applied to the lamp light emitting element (e.g., filament) part way through the power cycle. This causes a current pulse that can generate electromagnetic interference (EMI) and cause harmonic distortion (THD). Filtering elements (e.g., chokes) may be installed to reduce EMI and THD but such elements can increase the cost, weight and size of the system and can generate heat.
Another drawback of the phase-control clipping circuit is that the lamp may not operate properly when driven by a dimmer that delivers a clipped line voltage waveform to the lamp. The phase-control clipping circuit shown in FIG. 1 needs a sine wave voltage input for the phase setting components to operate correctly. If the line voltage has been clipped by a dimming circuit so that a sinusoidal voltage waveform is no longer presented to the lamp, the phase-control clipping circuit may not trigger the diac and triac as intended.
When the phase-control power controller is used in a voltage converter of a lamp, the voltage converter may be provided in a fixture to which the lamp is connected or within the lamp itself. U.S. Pat. No. 3,869,631 is an example of the latter, in which a diode is provided in an extended stem between the lamp screw base and stem press of the lamp for clipping the line voltage to reduce RMS load voltage at the light emitting element. U.S. Pat. No. 6,445,133 is another example of the latter, in which a voltage conversion circuit for reducing the load voltage at the light emitting element is divided with a high temperature-tolerant part in the lamp base and a high temperature-intolerant part in a lower temperature part of the lamp spaced from the high temperature-tolerant part.
Factors to be considered when designing a voltage converter that is to be located within a lamp include the sizes of the lamp and voltage converter, costs of materials and production, production of a potentially harmful DC load on a source of power for installations of multiple lamps, and the operating temperature of the lamp and an effect of the operating temperature on a structure and operation of the voltage converter.